Microﬂuidic Fabrication and Thermoreversible Response of Core Shell Photonic Crystalline Microspheres Based on Deformable Nanogels
基于可变形纳米凝胶的核/壳光子晶体微球的微流体制备及热可逆响应

abstract

Soft photonic crystals (PC) are more appealing due to the responsiveness of the building blocking-deformable nanoparticles to the external stimuli. In this report, we demonstrate, for the ﬁrst time, the generation of soft core/shell PC microspheres through a combination of a microﬂuidic technique, encapsulation of well-ordered temperature responsive polymer nanogels suspension, and photopolymerization of a transparent shell resin. This strategy not only ensures the monodispersity of core/shell PC microspheres, but also precisely controls their size, shell thickness, and optical properties by simply adjusting the ﬂow rate ratio and mass fraction of the nanogels. More interestingly, the intensity of the reﬂection spectra of the crystalline nanogel arrays in the core can be modulated reversibly by controlling the shell thickness or the temperature. As a result of their symmetric structure, the resulting PC microspheres exhibited excellent structural colors and photonic band gaps for normal incident light independent of the position on the spherical surface.Multifunctional PC microspheres can also be generated by simply dispersing functional species together with the nanogels. This core/shell PC microsphere with tunable shell thickness and reversible thermoresponse could be signiﬁcant for potential applications in the ﬁelds of chemical/biological sensors, display, encoding, and optical switching.
由于建立无变形性纳米粒子对外界刺激的响应性，软光子晶体更具吸引力。
在本报告中，我们首次通过微流体技术(有序温度响应性聚合物纳米凝胶悬浮液的封装)和透明壳树脂的光聚合作用，证明了软核/壳PC微球的生成。
该策略不仅可以确保核/壳PC微球的单分散性，而且可以通过简单地调节纳米凝胶的流速比和质量分数来精确控制它们的尺寸，壳厚度和光学性质。
更有趣的是，通过控制壳厚度或温度，可以可逆地调节核中结晶纳米凝胶阵列的反射光谱的强度。
由于它们的对称结构，所得到的PC微球对于垂直入射光表现出优异的结构颜色和光子带隙，而与球面上的位置无关。
通过简单地将功能性物质与纳米凝胶一起分散，也可以产生多功能PC微球。
这种具有可调壳厚度和可逆热响应的核/壳PC微球可能对化学/生物传感器，显示器，编码和光学切换领域的潜在应用具有重要意义。

1.INTRODUCTION

Colloidal photonic crystals (PC) are periodic dielectric arrangements of colloidal nanoparticles on the optical wavelength scale, which can produce unique optical properties, including photonic band-gaps and structural colors. These properties are widely applied in various ﬁelds including optoelectronic devices, sensors, displays, encoded particles, and optical switching.Generally, two types of photonic crystals, PC ﬁlms and PC microspheres, have been developed for meeting diﬀerent requirements. In comparison, PC microspheres are more appealing because PC microspheres can eﬀectively overcome the viewing angledependence of the structural color of PC ﬁlms due to their spherical symmetry, thereby broadening their perspective of applications.
胶体光子晶体（PC）是光学波长尺度上胶体纳米粒子的周期性介电排列，可以产生独特的光学性质，包括光子带隙和结构色。
这些特性广泛应用于各种领域，包括光电器件，传感器，显示器，编码粒子和光学开关。
通常，已经开发出两种类型的光子晶体，PC膜和PC微球，以满足不同的要求。
相比之下，PC微球更具吸引力，因为PC微球由于其球形对称性可以有效地克服PC膜结构颜色的视角依赖性，从而拓宽了它们的应用前景。

To prepare three-dimensional PC microspheres with closepacked or non-close-packed structures, both hard and soft monodispersed particles (e.g., polystyrene nanoparticles and poly(N-isopropylacrylamide) nanogels) are usually employed as building blocks.In contrast, the preparation process can be dramatically shortened to ∼10 s by using an optoﬂuidic technique,forming non-close-packed PC microspheres.To avoid the smaller order in the core region of PC microspheres, monodispersed PC microspheres or microcapsules can also be obtained by encapsulating or ﬁxing non-close-packed colloidal crystal suspensions, such as colloidal crystal arrays, into a transparent polymer shell assisted by the microﬂuidic technique. By encapsulating hard magnetic colloids into polymer microcapsules, PC microcapsules can be prepared, showing various structural colors when an external magnetic ﬁeld is applied.Yet, PC microspheres based on hard particles are diﬃcult to functionalize and lack the responsiveness to external stimuli, resulting in limited applications. On the contrary, soft colloids, especially those with a non-close-packed structure, show signiﬁcant potential in various ﬁelds such as sensors, displays, and microlenses, due to their obtainable crystalline structure at a low volume fraction of soft particles, unusual phase behaviors, and stimuli-response induced by their inherent soft nature.Up to now, there are only a few reports on the preparation of PC microspheres based on soft colloids. Thus, it still remains a challenge to generate monodispersed multifunctional polymer PC microspheres based on soft colloids in a fast and facile manner.
为了制备具有紧密包装或非密堆积结构的三维PC微球，通常使用硬和软单分散颗粒（例如，聚苯乙烯纳米颗粒和聚（N-异丙基丙烯酰胺）纳米凝胶）作为结构单元。
相比之下，通过使用光学流体技术，可以将制备过程大大缩短到~10秒，形成非密堆积的PC微球。
为了避免PC微球核心区域的较小顺序，单分散的PC微球或者微胶囊能够通过封装或者组装非密堆积胶体晶体悬浮液（比如胶体晶体序列）到由微流体辅助的透明聚合物壳中来获得。
通过将硬磁性胶体包封到聚合物微胶囊中，可以制备PC微胶囊，当施加外部磁场时显示出各种结构颜色。
然而，基于硬颗粒的PC微球体难以功能化并且缺乏对外部刺激的响应性，导致应用受限。
相反，软胶体，特别是那些具有非密堆积结构的胶体，在传感器，显示器和微透镜等各种领域中显示出显着的潜力，因为它们在软颗粒的低体积分数下可获得晶体结构，异常相行为，以及由其固有的软性引起的刺激反应。
到目前为止，关于基于软胶体的PC微球的制备仅有少数报道。因此，以快速且容易的方式生成基于软胶体的单分散多官能聚合物PC微球仍然是一个挑战。

Colloidal crystal arrays (CCAs) based on nanogels were ﬁrst developed by Asher’s group, but this system needs a complex pretreatment process for the puriﬁcation of nanogels, such as dialysis, ion exchange, etc. The crystalline structure of CCAs also suﬀers from external interferences such as vibration and ion strength. CCAs tend to lose stability in an aqueous medium containing electrolytes and other cosolutes or in external ﬁelds (e.g., shearing or electric ﬁelds). To improve the stabilities of the CCAs, most of the soft colloidal crystals have been prepared in the format of ﬁlm or bulk, followed by a post-crosslinking process.Alternatively, encapsulation of CCAs based on polystyrene nanoparticles within a spherical shell of low permeability of ions and molecules can preserve the ordered structures and aﬀord long-time stabilities to the CCAs.
基于纳米凝胶的胶体晶体阵列（CCAs）首先由Asher集团开发，但该系统需要复杂的预处理工艺来净化纳米凝胶，如透析，离子交换等。
CCA的晶体结构也受到外部干扰（如振动和离子强度）的影响。
CCA倾向于在含有电解质和其他cosolute的水性介质中或在外部场（例如剪切或电场）中失去稳定性。
为了提高CCA的稳定性，大多数软胶体晶体已经以薄膜或块状形式制备，然后进行后交联过程。
或者，在离子和分子的低渗透性球形壳内，基于聚苯乙烯纳米颗粒的CCA的包封可以保持有序结构和对CCA的长期稳定性。

Herein, we propose a facile and robust strategy to prepare monodispersed thermally responsive PC microspheres by using a microﬂuidic technique consisting of a co-ﬂow and ﬂowfocusing system, combined with an evaporation-induced crystallization technique.As shown in Figure 1, aqueous suspensions containing crystalline nanogel arrays were used as the inner ﬂuid. Notably, the well-ordered structures can be maintained during the formation of PC microspheres owning to the high viscosity and electrostatic interactions among the nanogel particles.The photocurable monomer, ethoxylated trimethylolpropane triacrylate (ETPTA) using a UV-sensitive initiator, was used as the middle phase (oil phase). Besides providing a spherical conﬁning environment for the ordering of crystalline nanogel arrays, the polymeric shell can also protect the encapsulated crystalline nanogel suspension and endow additional properties. The outer phase is an aqueous solution containing poly(vinyl alcohol) (PVA), which acts as surfactant to stabilize the interface between the middle oil and the outer aqueous phase. The water (suspension of crystalline nanogel arrays)/oil/water (aqueous solution of PVA) double emulsion droplets were generated in a microﬂuidic device. After the emulsion droplets passed downstream through the UV irradiation region, the middle oil phase was photopolymerized to provide solidiﬁed photonic spheres. This strategy not only ensures the monodispersity of the core/shell PC microspheres, but also allows us to precisely control the size, shell thickness, and optical properties (e.g., photonic band-gap and structural color) by simply adjusting the ﬂow rate ratio and mass fraction of the nanogels (φNP) in the core of the microspheres. More interestingly, the intensity of the reﬂection spectra of the crystalline nanogel arrays in the core can be modulated reversibly by controlling the shell thickness or the temperature.
在这里，我们提出了一种简便而稳健的策略，通过使用由流体和流体聚焦系统组成的微流体技术，结合蒸发诱导结晶技术，制备单分散热响应PC微球。
如图1所示，含有结晶纳米凝胶阵列的含水悬浮液用作内部流体。
值得注意的是，在PC微球的形成过程中可以保持良好有序的结构，这是由于纳米凝胶颗粒之间的高粘度和静电相互作用。
使用UV敏感引发剂的光固化单体乙氧基化三羟甲基丙烷三丙烯酸酯（ETPTA）用作中间相（油相）。
除了为结晶纳米凝胶阵列的排序提供球形限制环境外，聚合物壳还可以保护包封的结晶纳米凝胶悬浮液并赋予其它性能。
外相是含有聚（乙烯醇）（PVA）的水溶液，其用作表面活性剂以稳定中间油和外水相之间的界面。
在微流体装置中产生水（结晶纳米凝胶阵列的悬浮液）/油/水（PVA的水溶液）双乳液液滴。
在乳液液滴向下游通过UV照射区域后，中间油相被光聚合以提供固化的光子球。
这种策略不仅可以确保核/壳PC微球的单分散性，而且还可以通过简单地调节流速比和微球核心中的纳米凝胶的质量分数 (φNP)来精确控制尺寸，壳厚度和光学性质（例如，光子带隙和结构颜色）。
更有趣的是，通过控制壳厚度或温度，可以可逆地调节核中结晶纳米凝胶阵列的反射光谱的强度。

2.RESULTS AND DISCUSSION

2.1.Synthesis Strategy of the PC Microspheres Based on Polymer Nanogels.

基于聚合物纳米粒子的PC微球合成策略。

To generate the PC microspheres, we ﬁrst prepared monodispersed poly(N-isopropylacrylamide-coacrylic acid) (PNIPAm-AAc) nanogels by using a conventional precipitation polymerization approach.Figure 2a shows the TEM image of the obtained nanogels. Clearly, the nanogels have a uniform size and a narrow size distribution (also see the results of dynamic light scattering (DLS), SEM, and AFM results in Figure S2 of the Supporting Information (SI)). The size of the nanogels was found to be ca. 300 nm in an aqueous solution by DLS and ca. 130 nm after the samples were freeze− dried and measured by TEM investigation. Without any pretreatment, the aqueous suspension of the resultant PNIPAm-AAc nanogels was subsequently used to form wellordered crystalline structures through solvent-evaporation-induced crystallization, resulting in the self-assembly of nanogels and the formation of colloidal crystal arrays.The ordered crystalline structures were formed through the Coulombic repulsion of the charged PNIPAm-AAc nanogels. After microﬂuidic generation and followed by photopolymerization, the obtained PC microspheres showed typical core/shell structures (Figure 2b) and good monodispersity (PDI < 5%). The size of the PC microspheres and the shell thickness can be easily tuned by changing experimental parameters during microﬂuidic processing (Figures 3b−e and S3, SI). We note that it is diﬃcult to characterize the well-ordered structures of crystalline nanogel arrays in PC microspheres (for example, by SEM or TEM) since the water was removed from the nanogel suspension during the process of freeze−drying, resulting in dried nanogels precipitated on the inner surface of the polymer shell (Figure 3f).
为了生成PC微球，我们首先使用常规的沉淀聚合方法制备单分散的聚（N-异丙基丙烯酰胺 - 共聚丙烯酸）（PNIPAm-AAc）纳米凝胶。
图2a显示了所得纳米凝胶的TEM图像。显然，纳米凝胶具有均匀的尺寸和窄的尺寸分布（也参见支持信息（SI）的图S2中的动态光散射（DLS），SEM和AFM结果）。
发现纳米凝胶的尺寸为约300nm（在水溶液中通过DLS）和约130nm（将样品冷冻干燥后并通过TEM研究测量）。
在没有任何预处理的情况下，随后使用所得PNIPAm-AAc纳米凝胶的水性悬浮液通过溶剂蒸发诱导的结晶形成良好的晶体结构，导致纳米凝胶的自组装和胶体晶体阵列的形成。
通过带电的PNIPAm-AAc纳米凝胶的库仑排斥形成有序的晶体结构。
在微流体产生和随后光聚合之后，获得的PC微球显示出典型的核/壳结构（图2b）和良好的单分散性（PDI <5％）。
通过在微流体处理期间改变实验参数，可以容易地调节PC微球的尺寸和壳厚度（图3b-e和S3，SI）。
我们注意到，由于在冷冻干燥过程中从纳米凝胶悬浮液中除去水，因此难以表征PC微球中结晶纳米凝胶阵列的良好有序结构（例如，通过SEM或TEM），从而产生干燥的纳米凝胶。沉淀在聚合物壳的内表面上（图3f）。

When the mass fraction (φNP) of the crystalline nanogel arrays in suspension had a certain value, the crystalline nanogel arrays exhibited varied and brilliant structural colors (Figures 2c−e and S4, SI). Such varied structural colors only depend on the φNP of the crystalline nanogel arrays. For example, three typical structural colors, blue, green, and pink, originated from the PC microspheres containing 4.2%, 3.2%, and 2.2% of nanogels, respectively. These naked-eye visible structural colors can be further veriﬁed by ﬁber optic spectroscopy. The single sharp reﬂection peaks based on the Bragg diﬀraction of wellordered crystalline nanogel arrays were observed at wavelengths of 487, 538, and 591 nm, which are the maximum reﬂection peaks of a blue, green, and pink color, respectively (Figure 2f). Polymerized ETPTA as a shell layer exhibits a good transparency (the eﬀect of the shell thickness on the transparency and absorption can be found in Figure S5, SI), implying no inﬂuence on the display of structural colors of the core. However, the intensity of the reﬂection spectra would be inﬂuenced by the thickness of the polymeric shell, providing an approach to modulate the optical properties of the PC microspheres (for a further discussion, vide infra).
当悬浮的结晶纳米凝胶阵列的质量分数（φNP）具有一定值时，结晶纳米凝胶阵列表现出变化的和明亮的结构颜色（图2c-e和S4，SI）。
这种不同的结构颜色仅取决于结晶纳米凝胶阵列的φNP。例如，三种典型的结构颜色，蓝色，绿色和粉红色，分别来自含有4.2％，3.2％和2.2％纳米凝胶的PC微球。
这些肉眼可见的结构颜色可以通过光谱光谱进一步验证。在487,538和591nm的波长处观察到基于良好结晶的纳米凝胶阵列的布拉格衍射的单个急剧反射峰，其分别是蓝色，绿色和粉红色的最大反射峰（图2f）。
作为壳层的聚合ETPTA表现出良好的透明性（壳体厚度对透明度和吸收的影响可以在图S5，S1中找到），这意味着不会影响芯的结构颜色的显示。
然而，反射光谱的强度将受到聚合物壳厚度的影响，提供了调节PC微球的光学性质的方法（以进一步讨论，参见下文）。

Based on the fact that structural colors of the PC microspheres depend upon the φNP of the nanogels, it is indicative that the structural colors of the PC microspheres can be easily modulated by changing the concentration of the nanogel suspension. This can be explained by the Bragg law:

微信图片_20181009155155.png

where n is the eﬀective refractive index, d is the center-to-center space of the crystal planes next to each other, and θ is the viewing angle.Here, d is a key parameter to determine the position of the reﬂection peaks and structural colors. Apparently, d decreased with an increase of the nanogel concentration in the core of the PC microspheres, leading to a blue shift of the reﬂection peaks, and vice versa. This result is in accord with the case where hard nanoparticles were applied by Kim et al.

基于PC微球的结构颜色取决于纳米凝胶的φNP（质量分数）的事实，表明通过改变纳米凝胶悬浮液的浓度可以容易地调节PC微球的结构颜色。

这可以用布拉格定律来解释：

其中n是有效折射率，d是彼此相邻的晶面的中心的距离，θ是视角。

这里，d是确定反射峰的位置和结构颜色的关键参数。显然，d随着PC微球核心中纳米凝胶浓度的增加而降低，导致反射峰的蓝移，反之亦然。该结果与Kim等人应用硬纳米颗粒的情况一致。

2.2. Eﬀect of Shell Structures on the Optical Properties of PC Microsperes.

Polymer shells of the core/shell PC microspheres play important roles in the optical properties of the spheres.First, the solidiﬁed shell can endow PC microspheres with good mechanical strength and protect the core materials (crystalline nanogel suspension), preventing the escape of the nanogels and maintaining the ordered structures. Also, the shell inhibits evaporation of water when the encapsulated colloidal crystal arrays are exposed to the atmosphere. Second, the polymer shell can impact additional properties of the PC microspheres. For example, we show here that the shell thickness can be regulated by varying experimental parameters during the microﬂuidic processing, and the optical properties of the PC spheres can thus be modulated through simply varying the shell thickness of the PC microspheres. PC microspheres with a green structural color were taken as an example to investigate the eﬀect of shell thickness on their reﬂection spectra. As shown in Figure 4, when the average diameter of the cores increased from 137, 158, 175, to 198 μm, the relative average shell thickness decreased from 63, 43, 25, to 3 μm, respectively (inset micrographs of a, b, c, and d in Figure 4a). It was found that their relative intensities of reﬂection peaks at 517 nm (here, the φNP of nanogel was 3.5%) increased with the decrease of shell thickness, and vice versa (Figure 4b). A similar trend was also observed for the PC microspheres with blue and pink colors (Figure S6, SI). In addition, the eﬀect of core size on the reﬂection intensity was also investigated (Figure S7, SI). A very small change of the relative intensities was observed (ca. 0.2 au), indicating that the eﬀect of core size on the reﬂection intensity can be neglected. These results indicate that the reﬂection intensity of core/shell PC microspheres can be adjusted by changing the shell thickness (Figure 3). Although various core/shell PC microspheres have been reported,31 to the best of our knowledge, this strategy to modulate reﬂection intensities of PC microspheres by shell thickness has not been reported previously. It is thus a signiﬁcant advantage of the present work and is relevant for potential applications in the ﬁelds of optical sensors, color displays, etc.
核/壳PC微球的聚合物壳在球的光学性质中起重要作用。
首先，固化外壳可赋予PC微球良好的机械强度，并保护核心材料（结晶纳米凝胶悬浮液），防止纳米凝胶逸出并保持有序结构。
而且，当封装的胶体晶体阵列暴露于大气时，壳抑制水的蒸发。
其次，聚合物壳可以影响PC微球的附加性质。
例如，我们知道在微流体处理期间通过改变实验参数来能调节壳厚度，因此可以通过简单地改变PC微球的壳厚度来调节PC球体的光学性质。
以具有绿色结构颜色的PC微球为例，研究壳层厚度对其反射光谱的影响。
如图4所示，当芯的平均直径从137,158,175增加到198μm时，相对平均壳厚度分别从63,43,25减小到3μm（图4a中插入的显微图片a,b,c,d)
发现它们在517nm处的反射峰的相对强度（这里，纳米凝胶的φNP（质量分数）为3.5％）随着壳厚度的减小而增加，反之亦然（图4b）。
对于具有蓝色和粉红色的PC微球也观察到类似的趋势（图S6，SI）。此外，还研究了核的尺寸对反射强度的影响（图S7，SI）。
观察到相对强度的非常小的变化（约0.2au），表明可以忽略核的尺寸对反射强度的影响。
这些结果表明，核/壳PC微球的反射强度可通过改变壳厚度来调节（图3）。尽管已经报道了各种核/壳PC微球，但据我们所知，这种通过壳厚度调节PC微球反射强度的策略以前没有报道过。因此，它是当前工作的一个重要优势，并且与光学传感器，彩色显示器等领域的潜在应用相关。

2.3. Thermal Response of the PC Microspheres.PC微球的热响应

Temperature is the other important factor inﬂuencing the nanogels assembly and the corresponding structural color or Bragg wavelength of the PC microspheres. Here, we also took the PC microspheres with green structural color as the example to investigate their thermoresponsiveness. We found that the color of PC microspheres changed gradually from initially bright green to light green and the reﬂection peak was broadened correspondingly when the ambient temperature increased from 20 to 32 °C (Figure 5a, also Figure S8, SI). When the temperature further increased to 35 °C, the color of the PC microspheres became milky-white. On the contrary, a reversible response was observed when the temperature decreased from 35 to 20 °C, while the initial bright green color reappeared (Figure 5b). The reversibility can be repeated for at least 5 thermal cycles. Similar color variation was observed for the PC microspheres with a blue color (Figure S9, SI). Presumably, the thermoresponsiveness of the PC microspheres can be attributed to the size change of the nanogels and ordering of crystalline nanogel arrays during the temperature variation.

温度是影响纳米凝胶组件的另一个重要因素，以及PC微球的相应结构颜色或布拉格波长。在这里，我们还以具有绿色结构颜色的PC微球为例来研究它们的热响应性。
我们发现PC微球的颜色从最初的亮绿色逐渐变为浅绿色，当环境温度从20℃升高到32℃时，反射峰相应地变宽（图5a，也是图S8，SI）。
当温度进一步升高至35℃时，PC微球的颜色变为乳白色。相反，当温度从35℃降至20℃时，观察到可逆响应，而最初的亮绿色再次出现（图5b）。可逆性可重复至少5个热循环。对于具有蓝色的PC微球，观察到类似的颜色变化（图S9，SI）。据推测，PC微球的热响应性可归因于纳米凝胶的尺寸变化和温度变化期间结晶纳米凝胶阵列的排序。

PNIPAm hydrogels are typical thermoresponsible polymers with the special property of lower critical solution temperature (LCST, ∼32 °C). As the temperature is increased crossing the LCST of the nanogels, the nanogels start to shrink rapidly, resulting in a change in the diﬀraction color or Bragg diﬀraction wavelength. In this work, the thermoreversibility of the structural colors can be explained by the LCST of nanogels located in the core of PC microspheres. In the initial state of room temperature, swollen crystalline nanogel particles ﬁlled the core of PC microspheres, resulting in crystalline nanogel arrays formed under the conﬁnement of shell layer. In this case, the high intensities of reﬂection peaks can be attributed to wellordered crystalline nanogel arrays. When the temperature increased crossing the volume phase transition temperature, the nanogel particles experience a process of volume shrink (Figure S2d, SI).As a result, initial well-ordered crystalline nanogel arrays became a loose or disordered, milky ﬂuid. In this case, the shrink and motion of nanogels resulted in the disorder of crystalline nanogel arrays because the nanogels deviated from their crystalline lattice positions. Thus, the reﬂection peaks became wide and the intensities of the reﬂection peaks decreased, and tend to disappear with a further increase of temperature. However, when the temperature is decreased below the LCST of the nanogels, a well-ordered crystalline structure can spontaneously be recovered, showing their initial color and intensity of the reﬂection peaks. The PC microspheres with blue and pink colors also exhibit similar thermoresponsive behavior (Figures S10 and S11, SI), indicating the generality of this phenomenon. Thus, the PC microspheres can survive extensive physical or even chemical manipulation without degradation of the optical properties due to the remarkable tendency of the crystals to reorder.

PNIPAm水凝胶是典型的热敏聚合物，具有较低临界溶解温度（LCST，~32°C）的特殊性质。
随着温度越过纳米凝胶的LCST增加，纳米凝胶开始迅速收缩，导致差异颜色或布拉格反射波长的变化。
在这项工作中，结构颜色的热可逆性可以通过位于PC微球核心的纳米凝胶的LCST来解释。
在室温的初始状态下，溶胀的结晶纳米凝胶颗粒填充PC微球的核心，导致在壳层的结构下形成结晶纳米凝胶阵列。
在这种情况下，高强度的反射峰可归因于良好的结晶纳米凝胶阵列。当温度增加超过体积相变温度时，纳米凝胶颗粒经历体积收缩过程（图S2d，SI）。
结果，初始良好有序的结晶纳米凝胶阵列变成松散或无序的乳状流体。在这种情况下，纳米凝胶的收缩和运动导致结晶纳米凝胶阵列的无序，因为纳米凝胶偏离其晶格位置。
因此，反射峰变宽，反射峰的强度降低，并且随着温度的进一步升高趋于消失。
然而，当温度降低到纳米凝胶的LCST以下时，可以自发地恢复良好有序的晶体结构，显示它们的初始颜色和反射峰的强度。
具有蓝色和粉红色的PC微球也表现出类似的热响应行为（图S10和S11，SI），表明这种现象的普遍性。因此，由于晶体重新排序的显着趋势，PC微球可以经受广泛的物理或甚至化学操作而不会降低光学性质。

2.4. Viewing Angel Dependence of the Structural Colors for the PC Microspheres. PC微球结构颜色的视角依赖性.

An important property of PC microspheres is the viewing angle independence of their structural colors. In other words, the structural color of PC microspheres varies a little from diﬀerent viewing angles. Generally, the reﬂection colors depend on the angle between the incident light and the direction of view for any type of colloidal crystals.Yet, owning to their spherical symmetry structure, the PC microspheres can supply a wider angle than planar PC ﬁlms and reﬂect an identical color, independence of direction of view. To demonstrate the potential application of such arrays in optical devices and multipixel display materials, core/shell PC microspheres with intrinsic green color were dispersed in a poly(acrylic amide) hydrogel ﬁlm (Figure 6a). Signiﬁcantly, the composite ﬁlm shows a uniform color whether the view angle was 30° (Figure 6b), 60° (Figure 6c), or 90° (Figure 6d), indicating the independence nature with the viewing angle. Similar results were also obtained in the case of other PC microspheres with intrinsic blue and pink color (Figure S12, SI). Each nanogel in the array can be considered as an independent pixel unit at this time. The viewing angle independence should be ascribed to the spherical symmetry of the monodispersed PC microspheres.

PC微球的一个重要特性是其结构颜色的视角独立性。换句话说，PC微球的结构颜色从不同的视角变化很小。通常，反射颜色取决于入射光与任何类型的胶体晶体的视线方向之间的角度。
然而，由于它们的球形对称结构，PC微球可以提供比平面PC薄膜更宽的角度，并且反射相同的颜色，独立于视线方向。为了证明这种阵列在光学器件和多像素显示材料中的潜在应用，将具有内在绿色的核/壳PC微球分散在聚（丙烯酰胺）水凝胶膜中（图6a）。
显着地，无论视角是30°（图6b），60°（图6c）还是90°（图6d），复合膜都显示出均匀的颜色，表明视角的独立性。在具有固有蓝色和粉红色的其他PC微球的情况下也获得了类似的结果（图S12，SI）。此时，阵列中的每个纳米凝胶可被视为独立的像素单元。视角独立性应归因于单分散PC微球的球形对称性。

3. CONCLUSIONS

In summary, we demonstrate a new strategy to prepare monodispersed thermally responsive core/shell photonic crystal microspheres by using a microﬂuidic technique consisting of a co-ﬂow and ﬂow-focusing system and evaporation-induced crystallization. This strategy not only ensures the monodispersity of core/shell PC microspheres, but also precisely controls their size, shell thickness, and optical properties by simply adjusting the ﬂow rate ratio and mass fraction of the nanogels. As a result of their symmetric structure, the resulting PC microspheres exhibited excellent structural colors and photonic band gaps for normal incident light independent of the position on the spherical surface. We believe that the core/shell PC microspheres with tunable shell thickness and reversible thermoresponse could be signiﬁcant for potential applications in the ﬁelds of chemical/biological sensors, display, encoding, optical switching, etc. Moreover, functional species, e.g., ﬂuorescence dyes, quantum dots (Figure 6e,f), and magnetic nanoparticles, can be encapsulated within the microspheres to obtain multifunctional PC microspheres.

总之，我们展示了一种新的策略，通过使用由流体和流动聚焦系统和蒸发诱导结晶组成的微流体技术制备单分散热响应核/壳光子晶体微球。
该策略不仅可以确保核/壳PC微球的单分散性，而且可以通过简单地调节纳米凝胶的流速比和质量分数来精确控制它们的尺寸，壳厚度和光学性质。
由于它们的对称结构，所得到的PC微球对于垂直入射光表现出优异的结构颜色和光子带隙，而与球面上的位置无关。
我们相信具有可调壳厚度和可逆热响应的核/壳PC微球可能对化学/生物传感器，显示器，编码，光学转换等领域的潜在应用具有重要意义。此外，功能性物质，例如荧光染料，量子点（图6e，f）和磁性纳米粒子可以包封在微球内以获得多功能PC微球。

4. EXPERIMENTAL SECTION

4.1. Materials.

N-Isopropylacrylamide (NIPAM, purity ≥99%, Aladdin) was puriﬁed by recrystallization from n-hexane and dried under vacuum for 24 h. Acrylic acid (AAc, purity ≥98%, Sinopharm Chemical Reagent Co.) was puriﬁed by distillation at 398 °C/10 mmHg. Acrylamide (AAm, purity ≥98.5%, Sinopharm Chemical Reagent Co.), N,N-methylene bisacrylamide (MBA, Sinopharm Chemical Reagent Co., purity ≥98%), sodium dodecyl sulfate (SDS, purity ≥99%, Aldrich), ethoxylated trimethylolpropane triacrylate (ETPTA, purity ≥99%, Aldrich), poly(vinyl alcohol) (PVA, Mw = 13 000−23000, 87%−89% hydrolyzed, Aldrich), glycerol (purity ≥99%, Sinopharm Chemical Reagent Co.), potassium peroxydisulfate (KPS, purity ≥98%, Sinopharm Chemical Reagent Co.), 2-hydroxy-2methylpropiophenone (commercial name 1173, purity ≥99%, Aldrich), water-soluble CdSe quantum dots (Terminal groups were modiﬁed with −COOH, 8 mM, Wuhan Jiayuan Quantum Dots Co.), and ﬂuorescent dye Rhodamine B (Aladdin) were all used as received. Deionized water was produced from a Ruipure clean water system.
通过从正己烷中重结晶纯化N-异丙基丙烯酰胺（NIPAM，纯度≥99％，阿拉丁）并在真空下干燥24小时。通过在398℃/ 10mmHg下蒸馏来纯化丙烯酸（AAc，纯度≥98％，Sinopharm Chemical Reagent Co.）。丙烯酰胺（AAm，纯度≥98.5％，国药化学试剂公司），N，N-亚甲基双丙烯酰胺（MBA，国药化学试剂有限公司，纯度≥98％），十二烷基硫酸钠（SDS，纯度≥99％，Aldrich），乙氧基化三羟甲基丙烷三丙烯酸酯（ETPTA，纯度≥99％，Aldrich），聚（乙烯醇）（PVA，Mw = 13 000-23000，87％-89％水解，Aldrich），甘油（纯度≥99％，国药化学试剂） Co.），过二硫酸钾（KPS，纯度≥98％，国药化学试剂公司），2-羟基-2-甲基苯丙酮（商品名1173，纯度≥99％，Aldrich），水溶性CdSe量子点（端基被修饰）用-COOH，8mM，武汉嘉源量子点有限公司）和荧光染料罗丹明B（阿拉丁）均按原样使用。去离子水由Ruipure清洁水系统产生。

4.2. Methods.

Preparation of Nanogel and Multicolor PC Suspensions. Brieﬂy, 1 g of NIPAAm, 0.07 g of MBA, 0.03 g of SDS, and 0.08 g of acrylic acid were dissolved in 70 mL of deionized water. The solution was degassed and then bubbled with nitrogen for 1 h. Subsequently, the solution mixture was heated in an oil bath to ca. 70 °C. KPS (0.03 g) in 10 mL of water was dropwise added into the mixture to initiate the polymerization. The reaction was carried out for another 6 h with continuous stirring and nitrogen bubbling. After being cooled to room temperature, the resultant nanogel suspension was further stirred overnight. The cross-linking degree for all of the nanogel particles was kept constant at 5 mol % in our experiment while the molar ratio of MBA/NIPAAm was maintained at 0.05.
纳米凝胶和多色PC悬浮液的制备。将1g NIPAAm，0.07g MBA，0.03g SDS和0.08g丙烯酸溶解在70mL去离子水中。将溶液脱气，然后用氮气鼓泡1小时。随后，将溶液混合物在油浴中加热至约50℃。 70°C。向混合物中滴加KPS（0.03g）的10mL水溶液，引发聚合。在连续搅拌和氮气鼓泡下进行反应另外6小时。冷却至室温后，将所得纳米凝胶悬浮液进一步搅拌过夜。在我们的实验中，所有纳米凝胶颗粒的交联度保持恒定在5mol％，而MBA / NIPAAm的摩尔比保持在0.05。

In an oven at 70 °C, approximately 10 g of the nanogel suspension (without puriﬁcation) was evaporated to obtain a concentrated suspension with diﬀerent colors. The color variation of the suspensions depends upon the nanogel content. The nanogel content in the concentrated suspensions can be discretionarily regulated by controlling the evaporation time (8−12 h). As shown in Figure 2a, TEM images indicated that the nanogels self-assembled into nonclose-packed photonic crystals.
在70℃的烘箱中，蒸发约10g的纳米凝胶悬浮液（不含纯化物），得到具有不同颜色的浓缩悬浮液。悬浮液的颜色变化取决于纳米凝胶含量。通过控制蒸发时间（8-12小时），可以任意调节浓缩悬浮液中的纳米凝胶含量。如图2a所示，TEM图像表明纳米凝胶自组装成非封闭的光子晶体。

Microﬂuidic Fabrication of Core/Shell PC Microspheres. A specially designed microﬂuidic device consisting of a cylindrical injecting tube and a collection tube was inserted inside a square glass capillary as illustrated in Figure 1. The tip of the injection tube is tapered to an oriﬁce of ∼90 μm by heating and pulling a cylindrical capillary to a sharp point (using a Narishige PC-10 micropipet puller) and breaking the tip to the desired opening diameter (with a Narishige MF-900 microforge). The tapered collecting tube was produced by axial heating of the end of a 580 μm inner diameter capillary tube using the micropipet puller. The heating conditions were adjusted to produce tubes with an inner diameter of ∼300 μm at the capillary tip. The size of the emulsion droplets could be easily controlled by increasing the shear force of the continuous ﬂuid. This was accomplished by injecting the tip of the tapered injection tube inserted into the cylindrical collection tube.

微流体制备核/壳PC微球。如图1所示，在方形玻璃毛细管内插入一个由圆柱形注射管和收集管组成的特殊设计的微流体装置。
通过加热并将圆柱形毛细管拉到尖点（使用Narishige PC-10微量移液器拉拔器）并将尖端破碎至所需的开口直径（使用Narishige MF-900显微控制仪），注射管的尖端逐渐变细至约90μm的孔径。锥形收集管通过使用微量移液管拉出器对580μm内径毛细管的端部进行轴向加热而制成。
调节加热条件以在毛细管尖端处产生内径为~300μm的管。通过增加连续流体的剪切力可以容易地控制乳液液滴的尺寸。这是通过往圆柱形收集管中插入锥形注射管的尖端来实现的。

The colorful concentrated suspensions containing crystalline nanogel arrays were used as the inner ﬂuid. The photocurable monomer ETPTA (Figure S1, SI) with an UV-sensitive initiator was used as the middle ﬂuid (oil phase). The outer phase was an aqueous solution containing poly(vinyl alcohol) (PVA), which acts as surfactant to stabilize the interface between the middle oil and the outer aqueous phase. To increase the viscosity of the aqueous solution, a designed amount of glycerol was added to the PVA aqueous solution. Three types of ﬂuids were pumped into the microﬂuidic device, resulting in the formation of the water (suspension of crystalline nanogel arrays)/oil/water (aqueous solution of PVA) double emulsion droplets. The ﬂow rates of the three phases were adjusted to an appropriate condition to form stable double W/O/W emulsions under a dripping mode. The W/O/W double emulsion droplets were solidiﬁed by photopolymerization under radiation by a 400 W UV lamp (PORTA-RAY 400, Uvitron International, Inc.) for ∼40 s and collected in a beaker. The UV ignition distance between the samples and the UV lamp should be accurately controlled to avoid the formation of micropores on the shell surfaces as discussed in our previous report.
含有结晶纳米凝胶阵列的彩色浓缩悬浮液用作内部流体。具有UV敏感引发剂的可光固化单体ETPTA（图S1，S1）用作中间流体（油相）。外相是含有聚（乙烯醇）（PVA）的水溶液，其充当表面活性剂以稳定中间油和外水相之间的界面。为了增加水溶液的粘度，将设计量的甘油加入到PVA水溶液中。将三种类型的流体泵入微流体装置中，导致形成水（结晶纳米凝胶阵列的悬浮液）/油/水（PVA的水溶液）双乳液液滴。将三相的流速调节至适当的条件，以在滴加模式下形成稳定的双W / O / W乳液。 W / O / W双乳液液滴通过400W UV灯（PORTA-RAY 400，Uvitron International，Inc。）在辐射下光聚合固化约40秒并收集在烧杯中。应精确控制样品与紫外灯之间的紫外点火距离，以避免在壳表面形成微孔，如我们之前的报告中所述。

The nanogel suspensions containing the ﬂuorescent dye (Rhodamine B) or quantum dots (CdSe) were prepared to obtain functional PC microspheres. Typically, 50 μL of a Rhodamine B aqueous solution (0.05 wt %) was mixed with 2.5 mL of a concentrated nanogel suspension for 4 h. Alternatively, 2 mL of quantum dots aqueous solution (diluted to 0.32 μM) was mixed with 10 mL of the nanogel suspension before evaporation.
制备含有荧光染料（罗丹明B）或量子点（CdSe）的纳米凝胶悬浮液以获得功能性PC微球。通常，将50μL罗丹明B水溶液（0.05wt％）与2.5mL浓缩的纳米凝胶悬浮液混合4小时。或者，将2mL量子点水溶液（稀释至0.32μM）与10mL纳米凝胶悬浮液混合，然后蒸发。

Preparation of Hydrogel Films Containing PC Microspheres. A desired amount of PC microspheres was dispersed into a small amount of water in a glass Petri dish, followed by gentle stirring to spread the microspheres uniformly. Subsequently, 7 mL of an aqueous solution of AAm (4 mol/L), MBA (5 wt % compared to AAm), and photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-propanone (Darocur1173, Sigma-Aldrich) (1.5 wt % compared to AAm) was slowly added into the glass Petri dish. After 2 min, the solution mixture was polymerized under irradiation of an UV lamp (16 W) for 5 min.
含PC微球的水凝胶薄膜的制备。将所需量的PC微球分散在玻璃培养皿中的少量水中，然后温和搅拌以均匀地铺展微球。随后，7mL的AAm（4mol / L）水溶液，MBA（与AAm相比为5wt％）和光引发剂2-羟基-2-甲基-1-苯基-1-丙酮（Darocur1173，Sigma-Aldrich））（（与AAm相比为1.5重量％）缓慢加入玻璃培养皿中。 2分钟后，将溶液混合物在UV灯（16W）照射下聚合5分钟。

4.3. Characterization.

Atomic Force Microscopy (AFM). AFM was performed on a multimode atomic force microscope (SPA400, SEIKO) equipped with a Nanoscope III controller. Height proﬁles were recorded in the tapping mode at room temperature and against air. Samples were prepared by spin-coating nanogel suspensions (0.5 wt %) on a silicon wafer and air-dried for 8 h.
原子力显微镜（AFM）。 AFM在配备有Nanoscope III控制器的多模原子力显微镜（SPA400，SEIKO）上进行。在室温和空气中以轻敲模式记录高度曲线。通过在硅晶片上旋涂纳米凝胶悬浮液（0.5wt％）并空气干燥8小时来制备样品。

Dynamic Light Scattering (DLS) Analysis. A standard goniometer setup (ALV, Langen) was used to perform DLS measurements at various scattering angles. The sample temperature was adjusted with use of a thermostatted toluene bath acting as a temperature and refractive index-matching bath. The temperature was controlled by a PT100 thermoelement, which was placed in the toluene bath close to the sample position. This provided stability in temperature of ±0.1 K. The performed laser was a frequency-doubled Nd:YAG laser (Compass Series, Coherent) with λ = 532 nm providing a constant output power of 150 mW. The recorded intensity time autocorrelation functions were analyzed by inverse Laplace transforms (ILT), using the program CONTIN.
动态光散射（DLS）分析。使用标准测角仪设置（ALV，Langen）以各种散射角进行DLS测量。使用恒温甲苯浴作为温度和折射率匹配浴调节样品温度。通过PT100热电元件控制温度，将其置于靠近样品位置的甲苯浴中。这提供了±0.1K的温度稳定性。所执行的激光是倍频Nd：YAG激光（Compass Series，Coherent），λ= 532nm，提供150mW的恒定输出功率。使用程序CONTIN通过逆拉普拉斯变换（ILT）分析记录的强度时间自相关函数。

Optical Microscope. The formation of double emulsion droplets was monitored by an inverted optical microscope (IX71, Olympus) in bright-ﬁeld or phase-contrast modes. The images were captured by a high-speed CCD connected to the microscope. The diameters of the double emulsions and microspheres were analyzed by the Image J software.
光学显微镜。通过倒置光学显微镜（IX71，Olympus）在明场或相衬模式下监测双乳液液滴的形成。图像由连接到显微镜的高速CCD捕获。通过Image J软件分析双乳液和微球的直径。

Scanning Electron Microscope (SEM). A drop of diluted aqueous nanogel suspension was spin-coated on a clean silicon wafer and followed by freeze−drying. The morphology of the freeze−dried nanogel particles was imaged by SEM (Sirion 200, FEI). The accelerating voltage was 10 kV. Additionally, the freeze−dried microspheres were cut into two pieces by an ultrathin knife blade to observe the inner morphology.
扫描电子显微镜（SEM）。将一滴稀释的含水纳米凝胶悬浮液旋涂在干净的硅晶片上，然后冷冻干燥。通过SEM（Sirion 200，FEI）对冻干的纳米凝胶颗粒的形态进行成像。加速电压为10kV。另外，通过超薄刀片将冷冻干燥的微球切成两片，以观察内部形态。

Transmission Electron Microscope (TEM). The nonconcentrated nanogel suspensions were puriﬁed by dialysis for 14 days and then diluted to 3-fold. A drop of diluted nanogel suspension was spread on the TEM grid and freeze−dried. The sample was stained with 1 wt % phosphotungstic acid aqueous solution and then imaged by TEM (Tecnai G2 20, FEI).
透射电子显微镜（TEM）。通过透析纯化未浓缩的纳米凝胶悬浮液14天，然后稀释至3倍。将一滴稀释的纳米凝胶悬浮液涂布在TEM网格上并冷冻干燥。用1wt％磷钨酸水溶液染色样品，然后通过TEM（Tecnai G2 20，FEI）成像。

Fiber Optical Spectra. The reﬂection spectra of PC microspheres were measured by using a ﬁber optic spectrometer (USB4000, Ocean Optics Inc.) equipped with a DM2500P optical microscope. The temperature was controlled with a temperature controller (THMS600/CI94, Linkam). The rate of increasing/decreasing temperature was maintained at 2 deg/min.
光纤光谱。通过使用配备有DM2500P光学显微镜的光纤光谱仪（USB4000，Ocean Optics Inc.）测量PC微球的反射光谱。用温度控制器（THMS600 / CI94，Linkam）控制温度。升温/降温的速度保持在2度/分钟。

Microﬂuidic Fabrication and Thermoreversible Response of Core Shell Photonic Crystalline Microsph...